geom_imshow(): should render transparency for NaNs when all other pixel values are identical.

Currently, this code
```
image_data = np.array([
    [1.0, np.nan, 1.0],
    [np.nan, 1.0, np.nan],
    [1.0, np.nan, 1.0]
])

ggplot() + geom_imshow(image_data)
```

renders a solid all grey image.

Expected: transparency of NaN pixels. 